Practical experiences

Two remarkable events occurred in a Spanish wind farm is used in this section to analyze utility of the active and reactive formulations established in section 2 and their application for verifying grid code requirements. Those events are a three-phase balanced voltage dip and a two-phase voltage dip manifested at the connection point of a 660 kW rated power wind generator, with 690 V phase to phase nominal voltages.

Spanish grid code requirements in their two versions, O.P. 12.2 and O.P. 12.3, were not verified in the three-phase balanced voltage dip (fig. 9) and the installation was finally disconnected, mainly due to an excess of the supplied active current (figs. 10 and 11a). Comparison between active currents measured during the three-phase balanced voltage dip according to the two approaches included in section 2 (figs. 10 and 11a) shows traditional active currents used by the grid codes and fundamental positive-sequence active current have the same evolutions. And the same can be told for the traditional and positive-sequence reactive currents (fig. 12 and 13a). Active and reactive powers show the same tendencies and similar values with both theories (figs. 14 and 15, respectively). However,

while traditional active and reactive currents have different values in each phase, this one does not occur with the positive-sequence active and reactive currents; thus, the verification process of the grid code requirements is easier using the Unified Theory.

Fig. 9. Three-phase balanced voltage dip

Fig. 9. Three-phase balanced voltage dip

Fig. 11. Unified Theory's active currents: (a) total, (b) due to the active loads, (c) caused by the unbalances

-100

-150

-100

---

V"

V

— ii"

A

/

---

c

\

/

a, b

\

//

\

/

/

0,58 0,6 0,62 0,64 0,66 0,68 0,7 0,72 0,74 0,76 Time, t (s)

Fig. 13. Unified Theory's reactive currents: (a) total, (b) due to the reactive loads, (c) caused by the unbalances

Fig. 13. Unified Theory's reactive currents: (a) total, (b) due to the reactive loads, (c) caused by the unbalances

Time, t (s)

Fig. 15. Reactive powers: (a) traditional, (b) Unified Theory

Fig. 15. Reactive powers: (a) traditional, (b) Unified Theory

Spanish and German grid code requirements was verified by the wind farm in presence of the analyzed two-phase dip whether the Unified Theory is used. However, the application of the traditional theory is very complicated since the traditional active and reactive currents have different sign and value in each grid phases (figs. 16 and 18) and traditional active and reactive powers contain negative-sequence components. Unified Theory's positive-sequence active and reactive currents verify grid code requirements because their values are not increased during the fault (figs. 17a and 19a). Moreover, the maintenance of the positive-sequence reactive power is explained by an important consumption of the positive-sequence reactive current caused by the unbalances (fig. 19c), which compensate the increasing of the reactive current demanded by the grid (fig. 19b). Figure 20 shows how the duration of positive-sequence active power consumptions is less than the time period of the traditional active power consumptions and, thus, the accomplishment of the grid code requirements is improved. This fact occurs because a short positive-sequence active power delivery caused by the unbalances (fig. 21b). Difference between the traditional and the Unified Theory's reactive powers (fig. 22) defines the negative-sequence component of the reactive power which originates reverse magnetic fields and causes wind-generator malfunction. Positive-sequence reactive power is decreased by a strong reactive power consumption caused by the unbalances during the voltage dip (fig. 23b). This reduction of the positive-sequence reactive current supplied to the grid is convenient for the accomplishment of the grid code requirements.

The analysis of the two-phase voltage dip shows the Unified Theory is clearly better than the traditional theory for verifying the accomplishment of the grid code requirements, since that theory uses quantities more related with the active and reactive phenomena and it gives up additional information about those phenomena.

Time, t (sec.)

Fig. 16. Two-phase voltage dip

Fig. 16. Two-phase voltage dip

Fig. 18. Unified Theory's active currents: (a) total, (b) due to the active loads, (c) caused by the unbalances

1"H 200 c

-100

-200

a

b\

/

V

/

0,1 0,12 0,14 0,16 0,18 0,2 0,22 0,24 0,26 0,28 0,3 Time, t (s)

Fig. 20. Unified Theory's reactive currents: (a) total, (b) due to the reactive loads, (c) caused by the unbalances
Time, t (s)
Fig. 22. Unified Theory's active powers components: (a) due to the active loads, (b) caused by the unbalances
Time, t (s)
Fig. 24. Unified Theory's reactive power components: (a) due to the reactive loads, (b) caused by the unbalances
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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